Electrohydraulic servo valves are continuously acting valves that transform an electrical input signal into a stepless hydraulic output (flow or pressure). Servo valves can provide precise control of position, velocity, pressure and force. Most commonly they are mounted on linear or rotary actuators to transform an electrical control signal into linear or rotary motion output of an actuator e.g. for machinery position control. Many aircraft flight controls and engine controls are actuated using servo valves. For example, in rotorcraft such as helicopters both the main and tail rotor actuators may incorporate servo valves. Servo valve applications include aerospace, climate control, fluid & gas handling, hydraulics, pneumatics, and process control.
A typical electrohydraulic servo valve comprises an electrical torque motor as a first stage for controlling flow of a hydraulic fluid that drives a spool valve in a second stage. The second stage can in turn control hydraulic flow to an actuator driving a load. The hydraulic second stage typically comprises a spool and sleeve, for example a conventional four-way spool design in which the output from the valve, at a fixed pressure drop, is proportional to spool displacement from the null position. A hydraulic amplifier acts between the first and second stages to vary the piloting flow. The hydraulic amplifier can have various different designs, for example a jet pipe and receivers, a jet deflector and receivers, or a nozzle flapper.
In a jet pipe servo valve, hydraulic fluid at system pressure is directed through a jet tube towards receiver ports that are connected to the second stage spool. At the null position of the valve, the jet is directed equally between the receiver ports so that there is equal pressure on both sides of the spool. Deflection of the jet tube splits the flow unevenly between the receiver ports so as to create a differential pressure from one end of the spool to the other. The differential pressure created across the spool moves it in a direction opposite to the jet displacement.